1. Field of the Invention
The present invention relates to a method for driving a display apparatus having a display memory property.
2. Description of Related Art
As information equipment progresses, demands for display apparatus which consume low electric power and are shaped to be thin are increasing. Accordingly, research and development of display apparatus meeting the demands have been vigorously performed. In particular, a wearable personal computer (PC), an electronic organizer and the like are frequently used outdoors owing to their uses, and then they are expected to be power consumption-saving and space-saving. Accordingly, some products have been developed. Such products, for example, integrate display functions by means of thin type displays such as liquid crystal displays and coordinate inputting processing, and thereby can directly input the contents displayed on the displays by performing pressing operations with pens or fingers.
However, because many liquid crystal displays do not have the so-called display memory properties, it is required to continue to apply voltages to the liquid crystal displays during display periods. On the other hand, it is difficult for the liquid crystal displays having the display memory properties to ensure reliability in the case where uses in various environments such as a use for the wearable PC's are supposed, and consequently the liquid crystal displays having the display memory properties have not been put to practical use yet.
As one of the display systems which have the display memory properties and are formed in thin shapes and light weights, Harold D. Lees et al. proposes an electrophoresis display apparatus in their U.S. Pat. No. 3,612,758. Such a kind of the electrophoresis display apparatus utilizes an electrophoresis phenomenon in which electrophoresis particles are moved by Coulomb forces when an electric field is applied to a fluid dispersion in which the electrophoresis particles are dispersed in a dispersion medium. As one of the electrophoresis display apparatus, there is one having a configuration in which two electrodes, at least one of which is transparent, are opposed to each other with a predetermined gap between them and a fluid dispersion composed of a colored insulating liquid and a plurality of charged migration particles is sealed between the electrodes. When a voltage is applied between the electrodes, the charged migration particles are attracted to one of the electrodes according to the direction of the electric field. Then, an observer can see the color of the insulating liquid or the colors of the charged migration particles.
In such an electrophoresis display apparatus, the charged migration particles attracted to one electrode holds their spatial distribution state when the electric field in the fluid dispersion is made to be uniformly zero. Thus, a display image displayed by the spatial distribution state of the charged migration particles can be held. That is, the display device has the display memory property. Consequently, the electrophoresis display apparatus needs electric power only when display is rewritten, and the electrophoresis display apparatus is considered to be effective for reducing power consumption.
Active matrix system electrophoresis display apparatus utilizing the display memory properties are proposed in US AA2002021483 and US AA2002005832.
According to these proposals, in a reset action period, a reset voltage is written into each pixel electrode. Next, in a write period, a fixed voltage is applied to each pixel electrode for a period in proportion to a gradation value indicated by image data, or a voltage in proportion to a gradation value indicated by image data is applied to each electrode for a fixed period. After that, in a holding action period of a displayed image after a displayed sate has become a substantially desired gradation level, the potential differences between the pixel electrodes and a common electrode are turned to be zero for fixing the spatial distribution state of the electrophoresis particles by turning the electric field applied to the electrophoresis particles to be zero.
However, according to a result of research of the present inventor, in a drive of the conventional electrophoresis apparatus, when the potential differences between the pixel electrodes and the common electrode are abruptly turned to be zero as the holding action, an anti-electric field having a reversed polarity to the write voltage is generated in a liquid layer. When threshold value characteristics of the charged migration particles are insufficient, the charged migration particles are moved by the anti-electric field, and then a desired display state cannot be held. Thus, the conventional electrophoresis apparatus have a problem of an inhibition of the display memory property.
A residual direct current (DC) is regarded as one of the causes of the problem. Hereinafter, the residual DC will be described in detail. In a method of driving the electrophoresis display apparatus, as an example of the drive method is shown in a timing chart of FIG. 3, each of fields having certain time lengths is utilized to perform reset actions, write actions and holding actions for control of image display. The abscissa axis of the graph shown in FIG. 2A is a time axis, and the ordinate axis of the graph is a voltage axis. In FIG. 2A, a solid line indicates an example of a voltage to be applied to a certain pixel of the electrophoresis display apparatus from the outside, and a broken line indicates an effective voltage applied to the fluid dispersion actually at the time of the application of the voltage applied from the outside. Moreover, FIG. 2B shows an optical response at that time. In FIGS. 2A and 2B, a reference mark (A) designates a reset action period, a reference mark (B) designates a write period, and a reference mark (C) designates a holding action period. In the electrophoresis display apparatus, regions having different electrical time constants exist in the device, and electric charges are stored in interfaces between the regions. Consequently, DC components are generated. Generally, the DC components are called as the residual DC's. When the residual DC's are stored, for example, even if the application voltage to a pixel electrode is made to be zero as shown by the solid line in the holding action period as designated by (C) in FIG. 2, an anti-electric field having a reversed polarity to the applied voltage is sometimes generated in the fluid dispersion as shown by the broken line. Because a part of the charged migration particles is written back by receiving the operation of the anti-electric field, the holding of the image is not performed sufficiently.